As flexible barrier packages have continued to evolve and gain consumer acceptance in the world market, manufacturers of heat sealable packaging films have developed many specialty products with consumer convenience and practicality in mind. The easy open package is one such innovation. These types of film products are designed to provide hermetic heat seals with a calculated strength. An ideal product of this kind will produce a heat seal strength low enough to be opened by pulling apart manually without the need for any special tools. Additionally, the seals must be strong enough to maintain the integrity of the package during transportation and storage. Film packages with frangible seals are another application for this same easy open technology.
Existing methods of producing easy open flexible packages include sealing together two different types of films under carefully controlled conditions. The resulting heat seal is a thermoplastic adhesive bond that can be pulled apart. Such an easy open seal may be described as an "adhesive rupture" mechanism. An example of such a seal is the bond that may be formed between film web of low density EVA polyethylene and a high impact styrenes. Sealing a low density polyethylene film to a rubber modified high density polyethylene film is another such example of an adhesive rupture peel seal. Peelable heat seals of this type require the use of two distinct materials and possible package configurations are therefore limited. The obtained peel strength may also be overly sensitive to the conditions used to produce the seal. Peelable seals of this kind may also fail to provide required resistance to chemical attack.
Still another method of producing peelable or frangible seals involves the use of heat activatable adhesive coating. The adhesives are applied to the film substrate as water based liquid emulsions or solvent based solutions, then dried to form a film. Typically, flexographic or gravure process is employed and therefore these systems may also be applied as patterned zone coatings. An example of such a system is the "Adcote" line of coatings manufactured by Morton International of Chicago, Ill. Other systems may be applied as 100% solids using hot melt application equipment. These heat activatable adhesives form peelable or frangible seals when heat sealed to various compatible film materials. All peelable seals of this type lack resistance to chemical attack by solvents and other aggressive chemicals.
Yet another method of producing peelable or frangible heat seals involves the use of "cohesive rupture" films. These films are capable of forming peelable seals when sealed to themselves or to a standard packaging film fabricated from a compatible polymer. The peel mechanism itself is created at the time of extrusion. The film sealant layer is comprised of a homogenized blend of various heat-sealable, non-heat-sealable, and compatible polymers that will not form a solution. The layers of polymer matrix immediately below the point of conversion with another film in the heat seal area has a calculated cohesive strength. As the heat seals are pulled apart, the polymer matrix splits. The strength of the obtained heat seal may be altered by the exact formula percentages utilized in the homogenized blend. A further benefit of such a system is the visible evidence that a hermetic seal has been produced as witnessed by the transfer of material upon opening. One such peelable film is manufactured by UCB, S.A., Bruxelles, Belgium under the trade name "Rayopeel". Rayopeel is comprised of four separate polymer components. The first component is a heat-sealable rigid olefinic polymer. A non-heat sealable polymer, which does not form a solution with the first component polymer is chosen for the second component. The third component is a branched olefinic polymer and the fourth component is a polymer that is compatible with all the polymers chosen to represent the first three components. U.S. Pat. No. 3,879,492 to Bontinick describes the composition of the Rayopeel film in detail, which is incorporated herein by reference. The film reliably produces peelable seals that fall within a narrow and predictable range of force. Additionally, the product has a wide process window that will produce closely repeatable results. The material has also proven to be highly resistant to chemical attack and energy deterioration. The single most limiting aspect of this type of peelable system is the requirement to compromise the entire package strength in order to integrate the peelable or frangible seal function.